The present invention relates to a method for producing semiconductor crystals which are highly transparent in the one to three micron (.mu.) spectral range.
The method described herein comprises generally the introduction of donors into phosphorus and/or arsenic containing semiconductor compounds for the purpose of changing the transparency or electrical conductivity thereof. Classes of semiconductor compounds to which the invention may have particular application include chalcopyrite structured semiconductors formed of a Group IIB element (from the periodic table of elements column headed by zinc) and a Group IVB element (column headed by carbon) combined with phosphorus and/or arsenic, and the semiconductor compounds formed of a Group IIIB element (column headed by boron) combined with phosphorus and/or arsenic.
A compound of special interest is zinc germanium di-phosphide (ZnGeP.sub.2), a ternary chalcopyrite semiconductor, because of its characteristic non-linear optical properties and relatively wide bandgap which render it a prime candidate as a tunable infrared (IR) laser when used in an optical parametric oscillator. However, characteristic lack of transparency in the 1-3 .mu. wavelength range of ZnGeP.sub.2, as well as of other semiconductors enumerated herein, has severely limited utility in applications requiring short wavelength pump lasers such as a Nd:YAG laser at 1.06 .mu. or a Ho:YLF laser at 2.05 .mu., or a Tm:Ho:YAG at 2.09 .mu., or other lasers running to 2.9 .mu., to pump an oscillator.
Single ZnGeP.sub.2 crystals of suitable size (.about.0.7.times.0.7.times.2.0 cm or larger) for use as IR transmitting non-linear optical elements in the 0.8-12 .mu. range may be successfully grown by one or more prior art methods, such as the horizontal or vertical Bridgman methods in which the crystal material is melted in a horizontal boat or a vertical cylindrical vessel which is seeded or tapers conically to a point at the bottom, and the boat or vessel is moved or lowered slowly into a cold zone to initiate crystallization at the seed or in the tip. Alternatively, a dynamic gradient furnace may be utilized in the Bridgman process; in such a furnace, via multiple heating elements, a pre-programmed thermal gradient moves along the axis of the furnace eliminating the requirement to move the vessel or boat. Intrinsic defects in the form of acceptors in the as-grown crystal, however, result in a high resistance p-type form which has a large photo-absorption coefficient of about 4-20 cm.sup.-1 (i.e. lacks transparency) at 1-3 .mu.. Successful operation of the oscillator requires the absorption oefficient to be about 0.1 to 0.01 cm.sup.-1, which is orders of magnitude lower than that of state of the art crystals. Annealing for .about.100 hours at 450.degree.-550.degree. C. was shown to reduce the absorption coefficient in the 1-3 .mu. range to about 4 cm.sup.-1 (Rud et al, Sov Tech Phys Lett 7, 72 (1981)), a yet unacceptably high value. Irradiation by 2 MeV electrons adequate to introduce 10.sup.17 -10.sup.18 cm.sup.-3 point defects reduces the absorption coefficient in small crystals below 0.1 cm.sup.-1 (Brudnyi et al, Phys Stat Sol (9) 50, 379 (1979)), but electron beam processing with 1-2 MeV electrons is limited by an inherently short electron penetration depth resulting in a crystal thickness limitation of about 3 mm.
The invention solves or significantly reduces in critical importance problems in the prior art as just described by providing a method for producing semiconductor crystals which are highly transparent in the 1-3 .mu. range. A principal feature of the invention resides in exposing a crystal to ionizing radiation to introduce defect donors in the crystal to compensate for acceptors in the as-grown crystal. Penetrating gamma radiation with an energy in excess of 106 electron volts is preferred in order to introduce donors uniformly throughout a large crystal. The gamma radiation produces energetic 10.sup.6 eV photo electrons which produce point defect donors for cancelling the acceptors. The radiation exposure time required is that which produces 10.sup.17 to 10.sup.19 cm.sup.-3 donors and a substantially transparent crystal.
The invention may find substantial use in tunable IR laser spectrometers wherein an increase in resolution of several orders of magnitude is provided over that of state of the art spectrometers. Since laser spectrometers produce highly collimated beams over long distances, spectrometry of the atmosphere may be a major application in the quantitative determination of pollution levels and the enforcement of quality standards.
It is therefore a principal object of the invention to provide transparent semiconductor crystals.
It is a further object of the invention to provide a method for producing semiconductor crystals which are highly transparent in the 1-3 .mu. spectral range.
It is yet another object of the invention to provide a method for producing transparent semiconductor crystals for use as IR transmitting non-linear optical elements in the 1-3 .mu. range.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.